1. Field of the Invention
The present invention relates to an image sensor that includes a photoelectric conversion element group on which color filters are arranged in a predetermined pattern. An image sensor according to the present invention can be applied to a charge-coupled device (CCD). Furthermore, the present invention relates to a color imaging apparatus including an image sensor.
2. Description of the Related Art
Image pickup tubes and solid-state image sensors are widely used for various imaging apparatuses. A color image sensor incorporated in a single tube or a single plate (collectively referred to as “single sensor”) type color imaging apparatus can be configured to constitute a color imaging unit by itself. Therefore, the single color image sensor does not require any color separation prism and does not require any positioning adjustment unlike a multiple-plate image sensor. The single color image sensor can downsize a color imaging apparatus and reduce electric power consumption. The single sensor type color camera incorporating a color CCD image sensor (solid-state image sensor) is widely used.
The above-described color image sensor includes color filters (which are generally referred to as “stripe filters” or “mosaic filters”) disposed on a predetermined light-receiving plane and is configured to perform color modulation (color filter processing) to obtain color information from a single light-receiving element.
For example, if a color image sensor is configured to include RGB 3-color filters in a predetermined regular pattern disposed on photoelectric conversion elements (pixels), each pixel can have unique spectral sensitivity. When an image of an object is captured by a color image sensor, the image signal includes dot sequential color information reflecting the above-described color filter array. Therefore, if the image signal is separable into signals corresponding to respective color filters according to the above-described predetermined array, color information can be obtained based on the separated signals.
A luminance signal (Y signal) is obtainable from RGB information. In other words, a luminance signal of one pixel can be obtained if information of at least three pixels (each of RGB pixels) is obtained. This type of color image sensor can capture a color image by itself although luminance resolution may slightly deteriorate.
There are various types of conventional color filters including three primary color filters (e.g., RGB stripe and Bayer-type RGB mosaic) as well as complementary color filters (e.g., YeMgCy stripe, YeMgCyW 4-color mosaic, and YeMgCyG 4-color mosaic).
However, a conventional color filter layout using a regular array (e.g., Bayer array) tends to generate spurious resolution (color moire) based on spatial sampling according to the array. Therefore, a conventional single sensor-type color imaging apparatus requires an optical low-pass filter capable of assuring image quality. However, the provision of such a filter deteriorates image resolution. Therefore, the conventional color filter layout (represented by the Bayer array) may not simultaneously satisfy two requirements, i.e., prevention of color moire and improvement of resolution.
As discussed in Japanese Patent Application Laid-Open No. 2000-299871, there is a conventional method for randomly arraying color filters to suppress generation of color moire or reduction in image resolution. However, according to the method discussed in Japanese Patent Application Laid-Open No. 2000-299871, an image may include noise components due to non-uniform spatial frequency (deviation in frequency).